This paper explores both "who" has driven up US prison populations in recent years and "why" this growth has occurred. At least since the early 1990s, the "who" appears to primarily be prosecutors. Crime and arrests have fallen, and the percent of felony cases resulting in admissions and time served once admitted have been flat. But the probability that an arrest results in a felony charge has gone up significantly. (Limitations in data prevent us from examining the role of filing decisions before 1994.)

As for the "why," this paper provides some evidence that, at least since the crime drop began, increases in prison spending appear to track increases in state budgets fairly closely, suggesting that increased fiscal capacity is an important causal factor. It also looks at the politics-of-crime theories and explains that all previous efforts are unsatisfactory because they have focused on state and federal actors. Prosecutors, who are driving prison growth, are county officials, and it is unclear that state- and national-level political theories explain more-local outcomes.